Medicare beneficiaries who did not enroll in Medicare Part D or Medicare Part B or Premium-Medicare Part A when first eligible because they were covered under a group health plan are granted a Special Enrollment Period for Medicare Part A, Medicare Part B, and Medicare Part D prescription drug coverage if they no longer have employer or group health coverage.

The Medicare Part B Special Election Period (SEP)

Medicare beneficiaries who qualify for a Special Enrollment Period (SEP) can enroll in Medicare Part B without incurring a late-enrollment penalty while they are still employed (or their spouse is still employed and up to eight (8) months after they no longer have employer coverage, change employer coverage, or are no longer working.

As reference, the government's Medicare.gov site states:

Special Enrollment Period (SEP) for the Working Aged and Working Disabled.

Individuals who do not enroll in Part B or premium Part A when first eligible because they were covered under a group health plan based on their own or a spouse's current employment (or the current employment of a family member, if disabled) may enroll during the SEP. The individual can enroll at any time while covered under the group health plan based on current employment, or during the 8-month period that begins the month the employment ends or the group health plan coverage ends, whichever comes first.

NOTE: Individuals with ESRD [End-stage Renal Disease or kidney failure] are not eligible to enroll during the SEP.

(please see: https://www.cms.gov/medicare/ eligibility-and-enrollment/ origmedicarepartabeligenrol/ index.html or
https://www.medicare.gov/ sign-up-change-plans/ get-parts-a-and-b/ when-sign-up-parts-a-and-b/ when-sign-up-parts-a-and-b.html)

The Employer/Union Group Health Plan (EGHP) Medicare Advantage or Medicare Part D Special Election Period (SEP)

Medicare beneficiaries can enroll in a Medicare Part D or Medicare Advantage plan anytime while covered under the group health plan based on current employment, or up to two months after the employment ends - or employer coverage ends - or the group health plan coverage ends, whichever comes first.

As further reference, the Medicare Part D Manual states:

SEP EGHP (Employer/Union Group Health Plan)

- An SEP exists:

• for individuals enrolling in employer/union group-sponsored Part D plans,
• for individuals to disenroll from a Part D plan to take employer/union-sponsored coverage of any kind, and
• for individuals disenrolling from employer/union-sponsored coverage (including COBRA coverage) to enroll in a Part D plan.

The SEP EGHP may be used when the EGHP [Employer/Union Group Health Plan] allows the individual to make changes to their plan choices, such as during the employer’s or union’s “open season,” or at other times the employer or union allows.

This SEP is available to individuals who have (or are enrolling in) an employer or union plan and ends 2 months after the month the employer or union coverage ends. [emphasis added]

The individual may choose the effective date of enrollment or disenrollment, up to 3 months after the month in which the individual completes an enrollment or disenrollment request. However, the effective date may not be earlier than the first of the month following the month in which the request was made. The effective date also may not be earlier than the first day of the individual’s entitlement to Medicare.

View a list of other situations causing a Special Election Period (SEP)

See also: "Is there a Special Enrollment Period (SEP) available to join or leave an Employer or Union Group Health Plan (EGHP)?"

Sources include:

Medicare Prescription Drug Benefit Manual, Chapter 3 - Eligibility, Enrollment and Disenrollment, Updated: August 19, 2011 (Revised:  November 16, 2011, August 7, 2012,August 30, 2013, August 30, 2014,July 6, 2015, September 1, 2015, September 14, 2015, December 30, 2015, May 27, 2016,August 25, 2016, June 15, 2017,July 31, 2018 & August 12, 2020),
(https://www.cms.gov/Medicare/Eligibility-and-Enrollment/MedicarePresDrugEligEnrol)

Medicare Managed Care Manual, Chapter 2 - Medicare Advantage Enrollment and Disenrollment, Updated: August 19, 2011 (Revised: November 16, 2011, August 7, 2012, August 30, 2013,August 14, 2014, July 6, 2015, September 1, 2015, September 14, 2015, December 30, 2015, May 27, 2016, August 25, 2016, June 15, 2017, July 31, 2018 & August 12, 2020),
(https://www.cms.gov/Medicare/Eligibility-and-Enrollment/MedicareMangCareEligEnrol)

https://www.cms.gov/ medicare/ eligibility-and-enrollment/ origmedicarepartabeligenrol/ index.html or https://www.medicare.gov/ sign-up-change-plans/ get-parts-a-and-b/ when-sign-up-parts-a-and-b/ when-sign-up-parts-a-and-b.html and https://www.govinfo.gov/content/pkg/FR-2020-06-02/pdf/2020-11342.pdf )